Recent studies on the mutagenicity of chemical carcinogens in various test systems have shown that there is a positive corelation between mutagenicity and carcinogenicity. Previous studies in Neurospora crassa show that carcinogenic compounds are mutagenic in N. crassa and that non-carcinogenic compounds are non-mutagenic. Our studies also indicate that potent carcinogens induce predominantly base-pair substitution mutations. It is necessary to extend our studies to determine whether chemical carcinogens cause similar spectra of genetic alterations both in wild-type and repair-deficient strains of N crassa. The ad-3 test system of N. crassa is used in this study. Conidia from a genetically marked two-component heterokaryon of both wild-type and repair-deficient strains are treated with chemical carcinogens. The treated and untreated conidia are analyzed for the presence of ad-3 mutants by the direct method. Based on the background counts and the number of mutants, the survival and the mutation frequency are determined. The ad-3 mutants isolated from forward mutation experiments are analyzed by a series of genetic tests.